The present invention relates to a roll-shaped image-receiving sheet for thermal transfer printing and a process for forming an image thereon which is used for forming an image and a letter.
There are known various thermal transfer printing methods in which an image and a letter are transferred from a thermal transfer sheet with a color transfer layer on a substrate sheet to a surface of an image-receiving sheet, while the thermal transfer sheet is heated by a thermal head from the back-side of the thermal transfer sheet onto an image-receiving sheet.
These methods are roughly divided into two processes of a sublimation type thermal transfer printing method and a fusion type thermal transfer printing method in accordance with a construction of a color transfer layer. Both thermal transfer printing methods make it possible to form a full color image on a surface of image-receiving sheet. A full color image is formed in the following manner. For example, three or four thermal transfer sheets, that is, a yellow sheet, a magenta sheet, a cyan sheet, and a black sheet, if necessary, are prepared to pile up the image of each sheet onto the same surface of an image-receiving sheet.
With development of various hardware and software in connection with multi-media, the thermal transfer printing method is adapted for a hard copy system of full color for computer graphics, a stationary image for a satellite communication, a digital image for CD-ROM or the like and an analog image for video or the like to enlarge the market of the system.
An image-receiving sheet for thermal transfer printing used in the above mentioned method have various concrete uses. The sheet is typically used as a proof sheet for printing, an output sheet for image, an output sheet for a draft and a design in CAD/CAM and the like, a sheet for medical analysis instruments such as a CT (computerized tomography) scanner, an endoscope camera and the like, an output sheet for measurement instrument, a sheet for substitution of instant photograph, an output sheet for a face photograph onto identification card(ID card), credit card or the like, and various cards such as a sheet for a composite photograph on a souvenir picture in an amusement park, a game center, a museum, an aquarium and the like.
Further, with the diversificasion of uses described above, various sheets of label type, seal type, post card type and the like are developed. Then, a roll-shaped image-receiving sheet for thermal transfer printing is used as an image-receiving sheet in which the printing surface area of an image can be freely adjusted.
In an image-receiving sheet for thermal transfer printing as described above, for example, in the sheet of label type or seal type, a half cut treatment is previously done on an image receiving part to take off the image formed part from the sheet or perforations are previously formed along the circumference of the image receiving part to take off the image formed part therefrom. Further, in the sheet of post card type, a column for indicating a post code or a position for putting a postage stamp thereon is previously printed. Therefore, it is necessary to form an image on a predetermined position on the sheet.
However, a conventional roll-shaped image-receiving sheet has a problem that a thermal transfer image is deviated from a printing portion such as the half cut portion, the perforation forming portion or the stamp putting portion.
FIG. 12 is a schematic side view illustrating a conventional method of forming an image on an image-receiving sheet for thermal transfer printing. In FIG. 12, at first, a first dye layer of a thermal transfer sheet 122 is put on an image receiving position on the image-receiving sheet 120 by a thermal head 123 on a circumferential position of a platen roller 125 to print the image of a first color on the sheet 120 while the sheet 120 and the layer 122 are held between the thermal head 123 and the rotating platen roller. After that, the thermal head 123 is separated from the circumferential surface of the platen roller 125. In this state, the sheet 122 is moved forward by one pitch. In a second process, the image-receiving sheet 120 having a first image is rewound by a carrying roller 124 and a second image of a second color onto a first image is formed. At this time, a registration between the second image and the first image is done by adjusting a returned amount of the sheet 120 by the carrying roller 124.
When an image-receiving sheet cut in a proper size is used, the registration of an image on the sheet can be done by adjusting a returned amount of the sheet 120 by the carrying roller described above. However, in case that the image-receiving sheet in the form of a roll is used, a tension exerted on the carrying roller changes when the sheet 120 is moved because a diameter of a roll of the sheet remarkably changes between the start of transfer of the sheet and the end of transfer thereof. As a result, the carrying roller 124 is rotated loss motion (slip) to generate a shift of a returned amount of the sheet.
Further, in case that image-receiving sheets having different thickness or slippery property is used, the carrying roller 124 is rotated in a slipping manner to generate a shift of a returned amount of the sheet.
If a returned amount of the sheet shifts, a position of a first image is not registered with a position of a second image or later images. As a result, an image formed by the first image, the second image or later images becomes blurred.
Further, when a position of an image to be formed is defined by a half cut treating, a printing or the like on a surface of an image-receiving sheet, a position to form an image on the sheet is deviated from a position of an image actually formed on the sheet. As a result, the sheet is hardly used.
As the related arts of the above-mentioned techniques, Japanese Laid-Open Publication No. 237691/1986, No. 198497/1987 and No. 890/1990 disclose an image-receiving sheet for thermal transfer printing in which a detection mark is formed on a back surface of the sheet. Japanese Utility Model Laid-Open Publication No. 8971/1988 disclose a transparent sheet for thermal transfer printing in which a transparent detection hole for indicating a position for thermal transfer printing is formed.
Accordingly, for settling the problem, the object of the present invention is to provide a roll-shaped image-receiving sheet for thermal transfer printing and a process for forming an image onto a roll-shaped image-receiving sheet for thermal transfer printing in which a thermal transfer image is registered with a part previously printed such as a half cut portion, perforation forming portion, a column for writing a postal code, and a position for putting a stamp, and each color image is registered with each other when each color image is formed to obtain a clear and high quality thermal transfer image.